Here are two versions of the same function to list all files in a directory tree.

The first one is recursive (calls itself while going through subdirectories) :
<?php
function rec_listFiles( $from = '.')
{
    if(! is_dir($from))
        return false;
    
    $files = array();
    if( $dh = opendir($from))
    {
        while( false !== ($file = readdir($dh)))
        {
            // Skip '.' and '..'
            if( $file == '.' || $file == '..')
                continue;
            $path = $from . '/' . $file;
            if( is_dir($path) )
                $files += rec_listFiles($path);
            else
                $files[] = $path;
        }
        closedir($dh);
    }
    return $files;
}
?>

The second one is iterative (uses less memory) :
<?php
function listFiles( $from = '.')
{
    if(! is_dir($from))
        return false;
    
    $files = array();
    $dirs = array( $from);
    while( NULL !== ($dir = array_pop( $dirs)))
    {
        if( $dh = opendir($dir))
        {
            while( false !== ($file = readdir($dh)))
            {
                if( $file == '.' || $file == '..')
                    continue;
                $path = $dir . '/' . $file;
                if( is_dir($path))
                    $dirs[] = $path;
                else
                    $files[] = $path;
            }
            closedir($dh);
        }
    }
    return $files;
}
?>
The iterative version should be a little faster most of the time, but the big difference is in the memory usage.

Here is also a profiling function (works in php5 only) :
<?php
function profile( $func, $trydir)
{
    $mem1 = memory_get_usage();
    echo '<pre>-----------------------
Test run for '.$func.'() ...
'; flush();

    $time_start = microtime(true);
    $list = $func( $trydir);
    $time = microtime(true) - $time_start;

    echo 'Finished : '.count($list).' files</pre>';
    $mem2 = memory_get_peak_usage();

    printf( '<pre>Max memory for '.$func.'() : %0.2f kbytes
Running time for '.$func.'() : %0.f s</pre>',
    ($mem2-$mem1)/1024.0, $time);
    return $list;
}
?>